I'm sure somebody with a copy of COMSOL handy can answer this better than I can, but off the top of my head, there are two easy ways to do this in most CFD packages.

Depending on  how complicated your fluid model is (equation of state, constitutive relation, etc), one way is to simply copy the fluid model into another fictitious fluid model. So say one is water, and you call the other water2. Have the ambient fluid be water and the jet be water2.

The other way is to add a very small amount of a fictitious fluid to (again, say) the jet impinging on the cross-flow. Say one part in 10^6. You could even use the water2 defined above. Then you can color your entire sim by the amount of water2 present in water, scaling the colors to display the range from 0 to 1.0e-6. I've done this a number of times and it is a very pleasing and informative way to see what's going on for flows of the type you describe.

There may be other ways.

Yes it is possible. Once you solve the fluid dynamic equations, you can add other models. Thermal, chemical species, etc. The advantage is that you do not need to solve the  fluid again. You can use the available  flow solution as initial guess. And uncheck the fluid solution in solver step. of course if the density and viscosity are function of these scalars, a coupled solution must me considered.